1. Field of the Invention
The present invention relates to an information recording/reproducing apparatus for recording/reproducing information to or from an information recording medium such as an optical disk, and more particularly, to servo control such as focus control or tracking control.
2. Description of the Related Art
Up to now, a recording/reproducing apparatus using an optical disk as a recording medium performs servo control to cause a spot of laser light emitted from a laser light source of a pickup at the time of recording/reproduction to follow the center of a desirable track on the optical disk.
FIG. 6 shows a fundamental structure of a servo control device of an optical disk apparatus. When the optical disk apparatus starts a recording/reproduction operation, a spindle motor 2 rotates and then a laser light source (not shown) of a pickup 3 is turned on. After the laser light source is turned on, light reflected by the optical disk 1 is received by a plurality of photosensors (not shown) separated from one another in the pickup 3 and converted into current signals. The respective current signals detected by the plurality of photosensors are converted into a plurality of voltage signals by a plurality of I-V conversion circuits (not shown).
A focus error generation circuit 4 generates a focus error signal based on outputs from the plurality of I-V conversion circuits. The magnitude of the focus error signal indicates a deviation amount between a focal point of a light spot and a recording surface of the optical disk 1. An A-D conversion circuit 5 converts the focus error signal into a digital value. A focus phase compensation circuit 6 performs phase compensation processing on the focus error signal converted into the digital value.
An output from the focus phase compensation circuit 6 is sent to a focus servo gain circuit 7 which performs multiplication processing of a focus servo loop gain. An output from the focus servo gain circuit 7 is input into a focus actuator driver circuit 8. The focus actuator driver circuit 8 generates a drive signal for driving a focus actuator (not shown) of the pickup 3 based on the output from the focus servo gain circuit 7. An objective lens (not shown) of the pickup 3 is driven in a focus direction based on the drive signal.
Therefore, the focus control is performed such that the spot of the light emitted from the laser light source is focused on the recording surface of the optical disk 1.
In the same manner, a tracking error generation circuit 9 generates a tracking error signal based on the outputs from the plurality of I-V conversion circuits. The magnitude of the tracking error signal indicates a deviation amount in a radius direction of the optical disk 1 between the track center and the light spot on the optical disk 1. The tracking error signal is converted into a digital value by an A-D conversion circuit 10. As is the case with the focus control, a tracking phase compensation circuit 11 performs phase compensation processing.
Then, a tracking servo gain circuit 12 performs multiplication processing of a servo loop gain. A tracking actuator driver circuit 13 generates a drive signal for driving a tracking actuator (not shown) of the pickup. The tracking control is performed to follow the center of a desirable track on the optical disk 1 with a light spot based on the drive signal.
In each of the focus and tracking servo systems, the A-D conversion circuit used is normally a successive approximation A-D conversion circuit. In order to perform focus or tracking control with high accuracy, the A-D conversion circuit requires high accuracy, that is, high resolution. However, an increase in the resolution of the A-D conversion circuit results in an increase in cost of the A-D conversion circuit.
A configuration of a focus servo system such as shown in FIG. 7 is proposed in Japanese Patent Application Laid-Open No. 2001-307345. In FIG. 7, the elements which have the same functions as those shown in FIG. 6 are identified by like reference numerals. FIG. 7 shows in detail the focus servo system, in particular, a portion related to the generation of the focus error signal. The same structure can be applied to the tracking servo system.
When the recording/reproduction on an optical disk starts in the focus servo system, a spindle motor rotates and the optical disk is irradiated with a spot of light from a laser light source of a pickup. The focus/tracking servo control starts after the laser light source is turned on.
When the focus control is to be started, reflected light from the optical disk is received by a photosensor 14 which is separated into four light receiving portions (sensor portions) “A” to “D” in the pickup. Each of the light receiving portions “A” to “D” of the photosensor 14 generates a current signal corresponding to the amount of reflected light. The current signals are converted into voltage signals by I-V conversion circuits 15 to 18. The voltage signals are converted into one-bit digital signals by ΔΣ conversion circuits 19 to 22.
FIG. 8 shows a fundamental structure of each of the ΔΣ conversion circuits 19 to 22. In FIG. 8, input signals are the voltage signals from the I-V conversion circuits 15 to 18 shown in FIG. 7. Each of the ΔΣ conversion circuits 19 to 22 includes an integrator 28, a comparator 29 for binarizing an output from the integrator 28 to generate a one-bit output signal, a delay device 30 for delaying an output of the comparator 29 by a time corresponding to one sample, and a subtraction circuit 27 for performing subtraction between an output from the delay device 30 and the input signal.
The voltage signals generated based on the outputs from the four light receiving portions of the photosensor 14 are converted into one-bit digital data by ΔΣ conversion circuits 19 to 22 shown in FIG. 8. The respective one-bit data are input into decimation filters 23 to 26.
The decimation filters 23 to 26 convert the input one-bit data into data which have a sampling frequency lower than sampling frequency of the ΔΣ conversion circuits 19 to 22 and have multi-bit information. The multi-bit data obtained by the conversion are input into the focus error generation circuit 4. The focus error generation circuit 4 generates a focus error signal based on the plurality of multi-bit data output from the photosensor 14.
For example, when an astigmatic focus error detection method is used to generate a focus error signal, an arithmetic operation corresponding to (A+C)−(B+D) of the photosensor 14 is performed based on the multi-bit digital signals converted from the voltage signals output from the photosensor 14 to thereby generate the focus error signal. The focus phase compensation circuit 6 performs phase compensation processing on the generated focus error signal.
An output from the focus phase compensation circuit 6 is sent to the focus servo gain circuit 7, and after performing multiplication processing of a focus servo loop gain, an output from the focus servo gain circuit 7 is input into the focus actuator driver circuit 8. An output from the focus actuator driver circuit 8 drives the focus actuator in the pickup to perform the focus servo control.
Similarly, when the tracking servo control is to be performed, voltage signals generated based on the outputs from the photosensor 14 are input into the ΔΣ conversion circuits, and the outputs from the ΔΣ conversion circuits are input into the decimation filters. Then, a predetermined arithmetic operation is performed based on the outputs from the decimation filters to generate a tracking error signal, thereby performing the tracking servo control.
As described above, in the conventional servo control device, the outputs from the photosensor are converted into one-bit data by the ΔΣ conversion, and each of the focus error signal and the tracking error signal is generated based on the data obtained by converting the one-bit data into multi-bit data by the decimation filters. In such a configuration, an A-D converter with a high resolution is not necessary and servo control can be realized with high accuracy.
As described above, the conventional servo control device includes the ΔΣ conversion circuit for each of the voltage signals generated by the separated portions of the photosensor and the decimation filters for converting the output of the ΔΣ conversion circuit into the multi-bit data. For example, when a focus error signal of the servo control device is to be generated by arithmetic operation of the four sensor outputs according to an astigmatic focus error detection method, four ΔΣ conversion circuits and four decimation filters are necessary.
Further, when a tracking error signal is to be generated by arithmetic operation of the eight sensor outputs according to a differential push-pull method, eight ΔΣ conversion circuits and eight decimation filters are necessary.
As described above, for example, when an astigmatic focus error detection method as a system of generating a focus error signal and a differential push-pull method as a system of generating a tracking error signal are adopted to perform focus servo control and tracking servo control, eight ΔΣ conversion circuits and eight decimation filters are necessary on the assumption that the ΔΣ conversion circuits and the decimation filters are commonly used for both the focus servo control and the tracking servo control.
Moreover, there is a possibility that the numbers of the necessary ΔΣ conversion circuits and the necessary decimation filters may increase depending on the system of generating a servo error signal. Therefore, it is necessary to provide a large number of decimation filters depending on the servo system, thereby increasing the circuit scale. In addition, there has been a problem that the production cost is increased by the increase in the circuit scale.